Youngkyu Sung

Characterizing and optimizing qubit coherence based on SQUID geometry

  1. Jochen Braumüller,
  2. Leon Ding,
  3. Antti Vepsäläinen,
  4. Youngkyu Sung,
  5. Morten Kjaergaard,
  6. Tim Menke,
  7. Roni Winik,
  8. David Kim,
  9. Bethany M. Niedzielski,
  10. Alexander Melville,
  11. Jonilyn L. Yoder,
  12. Cyrus F. Hirjibehedin,
  13. Terry P. Orlando,
  14. Simon Gustavsson,
  15. and William D. Oliver
The dominant source of decoherence in contemporary frequency-tunable superconducting qubits is 1/f flux noise. To understand its origin and find ways to minimize its impact, we systematically
study flux noise amplitudes in more than 50 flux qubits with varied SQUID geometry parameters and compare our results to a microscopic model of magnetic spin defects located at the interfaces surrounding the SQUID loops. Our data are in agreement with an extension of the previously proposed model, based on numerical simulations of the current distribution in the investigated SQUIDs. Our results and detailed model provide a guide for minimizing the flux noise susceptibility in future circuits.
arxiv pdf

Two-qubit spectroscopy of spatiotemporally correlated quantum noise in superconducting qubits

  1. Uwe von Lüpke,
  2. Félix Beaudoin,
  3. Leigh M. Norris,
  4. Youngkyu Sung,
  5. Roni Winik,
  6. Jack Y. Qiu,
  7. Morten Kjaergaard,
  8. David Kim,
  9. Jonilyn Yoder,
  10. Simon Gustavsson,
  11. Lorenza Viola,
  12. and William D. Oliver
Noise that exhibits significant temporal and spatial correlations across multiple qubits can be especially harmful to both fault-tolerant quantum computation and quantum-enhanced metrology.
However, a complete spectral characterization of the noise environment of even a two-qubit system has not been reported thus far. We propose and experimentally validate a protocol for two-qubit dephasing noise spectroscopy based on continuous control modulation. By combining ideas from spin-locking relaxometry with a statistically motivated robust estimation approach, our protocol allows for the simultaneous reconstruction of all the single-qubit and two-qubit cross-correlation spectra, including access to their distinctive non-classical features. Only single-qubit control manipulations and state-tomography measurements are employed, with no need for entangled-state preparation or readout of two-qubit observables. While our experimental validation uses two superconducting qubits coupled to a shared engineered noise source, our methodology is portable to a variety of dephasing-dominated qubit architectures. By pushing quantum noise spectroscopy beyond the single-qubit setting, our work paves the way to characterizing spatiotemporal correlations in both engineered and naturally occurring noise environments.
arxiv pdf

A tunable coupling scheme for implementing high-fidelity two-qubit gates

  1. Fei Yan,
  2. Philip Krantz,
  3. Youngkyu Sung,
  4. Morten Kjaergaard,
  5. Dan Campbell,
  6. Joel I.J. Wang,
  7. Terry P. Orlando,
  8. Simon Gustavsson,
  9. and William D. Oliver
The prospect of computational hardware with quantum advantage relies critically on the quality of quantum gate operations. Imperfect two-qubit gates is a major bottleneck for achieving
scalable quantum information processors. Here, we propose a generalizable and extensible scheme for a two-qubit coupler switch that controls the qubit-qubit coupling by modulating the coupler frequency. Two-qubit gate operations can be implemented by operating the coupler in the dispersive regime, which is non-invasive to the qubit states. We investigate the performance of the scheme by simulating a universal two-qubit gate on a superconducting quantum circuit, and find that errors from known parasitic effects are strongly suppressed. The scheme is compatible with existing high-coherence hardware, thereby promising a higher gate fidelity with current technologies.
arxiv pdf